With his announcement Mr. Katainen opened himself up for a job in Brussels and dashed the hopes of his close political ally and party colleague Alexander Stubb to become the highest-profile Finn in the EU capital. Before Mr. Katainen revealed his plans, Mr. Stubb — an ultra-fit endurance-sports enthusiast — had been seen as the frontrunner to take Finland’s spot in the next European Commission, the EU’s executive.